Shrinkable thermoplastic films having a gas barrier have found many useful applications in packaging of meats, cheeses, poultry, and numerous other food and non-food products. There is always the search for improvement in these films to make them have better barrier properties, better abuse resistance, better tear resistance, improved clarity, and easier handling. One film of this type is a multi-layer film having layers of polyethylene/saran/polyethylene which is disclosed in U.S. Pat. No. 3,821,182 which issued on June 28, 1974 to William G. Baird, Jr. et. al. The shrink and abuse resistance of such a film is improved by irradiating the film to cross-link the polyethylene layers prior to heating and orienting the film by the trapped bubble technique.
Another film laminate that has been quite successful is that disclosed in U.S. Pat. No. 3,741,253 which issued on June 29, 1973 to Harri J. Brax et. al. Disclosed in this patent is a multi-ply laminate which has a first layer of cross-linked ethylene-vinyl acetate copolymer directly joined to a middle layer of a copolymer of vinylidene chloride which is joined to another ethylene-vinyl acetate copolymer layer. The ethylene-vinyl acetate copolymer (hereinafter EVA) layer has improved properties over the previously used polyethylene and, in the extrusion coating method used to produce the multi-layer film according to the Brax et. al. patent, the substrate EVA layer is preferably cross-linked by irradiation before the saran layer is extrusion coated thereon thus avoiding irradiation of the saran layer. Saran (vinylidene chloride copolymer) tends to discolor under high energy irradiation.
An alternate and successful multi-layer film where a hydrolyzed ethylene-vinyl acetate copolymer is used as a barrier layer instead of saran is disclosed in U.S. Pat. No. 4,064,296 which issued on Dec. 29, 1977, to Norman D. Bornstein et. al. A heat shrinkable multi-layer film is formed by coextruding the hydrolyzed ethylene-vinyl acetate copolymer (sometimes abbreviated "HEVA" or called ethylene-vinyl alcohol and abbreviated "EVAL" or "EVOH".) Since EVOH does not suffer from the effects of radiation a coextruded product such as EVA/EVOH/EVA can readily be cross-linked by irradiation before orientation.
Another way of improving the performance of packaging films has been to blend various polymers. In U.S. Pat. No. 3,090,770 which issued on May 21, 1973 to Razmic S. Gregorian, the blending of cross-linked polyethylene with non-cross-linked polyethylene is disclosed to improve the clarity of a film. Such blends were accomplished by using differing proportions of high, low and medium density polyethylene. This Patent also disclosed a cross-linked polyethylene; and, U.S. Pat. No. 3,118,866, which issued on Jan. 28, 1964 to the same inventor, is directed to an ethylene composition and the process of cross-linking by chemical means. The olefin polymers and copolymers have been particularly attractive because of low cost, availability, and wide range of satisfactory characteristics for packaging films.
Recently, the medium and low density linear polyethylenes have become commercially available and have begun to be used in a number of packaging applications. One of the early patents in this field is U.S. Pat. No. 4,076,698 which issued on Feb. 28, 1978 to Aurthur William Anderson and discloses an interpolymer composed of ethylene and monoalpha-olefinic hydrocarbons containing five to ten carbon atoms per molecule and the proportion of the mono-olefinic hydrocarbon being 3 to 7 percent of the weight of the interpolymer with a melt index from 0.3 to 20 and a density of 0.93 to 0.94. Linear polymers of this type are characterized by actually being an interpolymer or copolymer with another olefin and having a relatively straight molecular chain, that is, having a chain with no side branches or limited side branching. Low density versions of this type of film where density is in the range of 0.920 to 0.926 are produced by a low pressure process as opposed to the high pressure process which produces a branched, low density polyethylene. Linear low density polyethylene, abbreviated hereinafter as "LLDPE", has found many applications and uses as exemplified by U.S. Pat. No. 4,364,981 which issued on Dec. 21, 1982 to Jerome T. Horner and discloses an EVA/LLDPE/EVA structure as does also U.S. Pat. No. 4,399,180 which issued on Aug. 16, 1983 to William F. Briggs et. al. In U.S. Pat. No. 4,457,960 a multi-layer structure is disclosed of EVA/Saran/EVA-LLDPE-blend.
Still another polymeric material has more recently entered the market having different properties from the copolymers which comprise the LLDPE class of materials. These copolymers are known as very low density polyethylene (hereinafter abbreviated "VLDPE") and whereas conventional polyethylenes and LLDPEs have densities as low as 0.912, the VLDPE's currently on the market have densities below 0.910, specifically, 0.900 to 0.906, and it is thought that densities as low as 0.860 will be forthcoming. European patent application No. 120,503 (Union Carbide) has been published disclosing a method of making VLDPE. In "Plastics Technology" magazine for September 1984 at page 113, a news item entitled "Introducing Very Low Density PE" briefly described some of VLDPE's properties and stated that it's what the manufacturer "Calls an entirely new class of polyethylene, consisting of linear copolymers that can be produced at densities down to 0.89 or lower. What makes them special is an unique combination of properties in between those of standard PE's and polyolefinic rubbers". In the October 1984 issue of "Plastics Technology" at page 13 another article appeared entitled "New Kind of Polyethylene Combines Flexibility, Toughness, Heat Resistance". This article lists a number of the properties of VLDPE and compares them with EVA and states that uses for this material is for squeeze tubes, bottles, hoses, tubing, drum liners and film. VLDPE is also listed as having potential as an additive. It is expected to be used as a blending resin in high density polyethylene, polypropylene, EVA, and some EPR's, with all of which VLDPE is compatible. According to the article, the first two commercially available grades are from Union Carbide and are designated "DFDA-1137 NT7", which has a narrow molecular weight distribution, higher toughness, clarity, and gloss and FDA clearance for food contact. The other resin is DFDA-1138 which is aimed particularly at film, has a broad molecular weight distribution, and is superior in processability. On page 15 in the same article, it is stated that "the new resins have been injection molded, extruded, blow molded, and thermoformed on standard equipment". It is noted that blown film can be extruded on systems designed either for conventional LDPE or for LLDPE. However, the company generally recommends LLDPE-type screw designs in higher torque capability, especially with narrow-MWD grades. The article observes that the enlarged die gaps required by LLDPE are not required for VLDPE and that conventional blown film die gaps of 30-40 mil have proven satisfactory at blow up ratios of 2-3:1. For blown film, DFDA-1137 and 1138 are said to extrude much like 2-MI LLDPE or 0.5-MI LDPE. An article similar to the one in "Plastics Technology" appeared in the October 1984 issue of "Plastics World" at page 86.
In the above mentioned European patent application publication No. 120,503, published Oct. 3, 1984, a process for preparing very low density ethylene polymers in a fluidized bed is described. These ethylene polymers are classified as having a density of less than 0.91 grams per cubic centimeter and having a melt flow index which is preferably from 0.2 to 4.0.
Now, returning to the specific application of polymers to packaging film and receptacles made therefrom, a very successful and useful film is made according to the process shown in U.S. Pat. No. 3,741,253 mentioned above. A heat shrinkable bag can be made from such film which has wide application, particularly for meat, poultry, and dairy products. In fact, heat shrinkable polymeric films have gained rather wide spread acceptance for packaging meat, particularly fresh meat and processed meat. Bags made from the heat shrinkable film are supplied to a meat packer being sealed at one end with the other end open and ready to receive a meat product. After the cut of meat is placed in the bag, the bag will normally be evacuated and the open end of the bag closed by heat sealing or by applying a metal clip. This process is advantageously carried out within a vacuum chamber where the evacuation and application of the clip or heat seal is done automatically. After the bag is removed from the chamber it is heat shrunk by applying heat which process can be performed by immersing the filled bag into a hot water bath or conveying it through a hot air tunnel.
In the usual distribution chain, a whole primal or subprimal is packaged within shrink bags of this type. The meat within the bag will travel from a central slaughter house where it has been packaged to a retail supermarket where the bag will be opened and the meat will be cut for retail portions. Thus, the bags of this type must satisfy a number of requirements which are imposed by both the slaughter house or packing house and by the bag user. Furthermore, often the bag is placed in the show case at the retail supermarket for special promotions when a whole loin, for example, is to be sold to the consumer for his use. At this point, it is desirable to have an attractive package whereby there has been relatively complete shrinkage of the bag around the product so that the bag is not wrinkled and the blood and juices are not trapped in the folds of the wrinkles. Accordingly, it is one object of the present invention to provide a film and receptacle made therefrom which has improved shrink characteristics over bags used in the past.
Another important characteristic of a bag is the capability of the bag to physically survive the process of being filled, evacuated, sealed, closed, heat shrunk, boxed, shipped about the country, unloaded, and stored at the retail supermarket. This type of abuse rules out many polymeric films. Accordingly, it is another object of the present invention to provide a new combination of polymeric films which will withstand the abuse of packing, shipping, and storing.
Another feature required by bags used for the foregoing described application is that the bag must also be strong enough to survive the handling involved in moving package meat which may weigh 100 pounds or more or large chunks of cheese weighing 60 lbs. or more. In particular, when the chunk of meat or cube of cheese is pushed into the bag its bottom seal must withstand the force of the meat or cheese as it hits the seal. Also, in bags that are made by folding a sheet with the fold as the bottom of the bag and by sealing the sides, seal strength is quite an important factor. Accordingly, it is still another object of the present invention to provide a bag which has improved seal strength over previously available bags.
It is also very desirable for the bag to serve as a barrier against oxygen from the surrounding atmosphere which will detrimentally affect the fresh meat product. Accordingly, it is yet another object of the present invention to provide a flexible film product which will maintain during its packaging life time an effective barrier to gases and oxygen, in particular.
One of the more common hazards in packaging and distributing products in flexible packaging materials is the hazard of the material receiving a puncture which will release the vacuum inside the bag and allow oxygen to enter. Anything from the application of the clip to the presence of a bone in the meat can cause a puncture. Accordingly, it is an important object of the present invention to provide a film which has superior puncture resistance.